(1) Field of the Invention
The present invention relates to an easily openable packaging container and method of producing the same. Particularly, the present invention relates to an easily openable packaging container which, due to its ability of being strongly heat-sealed, has excellent sealing property, and as well has excellent easy openability, and to the method of producing the container. The easily openable packaging container may be suitably used for packaging foods or liquids, particularly for heat sterilization packaging.
(2) Description of the Related Art
To ensure hermetical packaging of foods or liquids, containers has been heat-sealed with a lid member after a content has been filled therein. However, the conventional heat-sealed packaging containers thus produced have a defect that if heat-seal strength is increased to improve their seal property, openability will be aggravated causing the necessity for cutting instruments to gain access to the content. On the other hand, when the openability is improved by choosing materials of containers and lid members which can provide a proper heat-seal strength, the seal properties of the obtained containers broadly varies depending upon the heat-seal conditions, such as temperature, pressure, and time, or owing to foreign matters such as the content sandwiched between the heat-seal surfaces, and uniform heat-seal strength therefore cannot be obtained, resulting in a decrease of the seal property. In the latter case, the packaging containers thus produced are inapplicable to packages in which the internal pressure increases at the time of heat sterilization such as boiling or retort treatment. That is, the conventional hermetically sealed containers, which is to be opened by peeling the lid member between their heat-seal surfaces, could not reconcile the antipodal objects, secure seal and easy openability, simultaneously.
In Japanese Utility Model Application Laid-open No. 148,742/1988 is disclosed a method for solving these problems by providing a circular notch to the under-lid of a multilayer lid to cut the under-lid at the time of peeling the lid, for the facility of peeling. The method, however, provides seal property by spacing out the notch and the outer edge of the under-lid, and after opening the containers, a part of the under-lid outside the notch remains protruding inside the opening. That is, the resulting aperture is a partial hole, and it is impossible to open the whole of the opening portion of the containers unlike the other conventional containers. The containers therefore encountered a problem that they are inapplicable to drinks or desserts to be drunk or eaten directly from the opened containers since the contents cannot be discharged readily.
In Japanese Pat. Applciation Laid-open No. 190,461/1986 is disclosed a method of attaining easy openability by choosing a resin material having a proper internal cohesive failure of the resin. The method however has problems that the seal layer resins are limited and the peeled surfaces of the containers are rough.
In Japanese Pat. Application Laid-open No. 193,848/1986 is disclosed a method of attaining easy openability by choosing a particular combination of resin materials for heat-sealing, i.e. a multilayer material including a blend layer of ethylene-vinyl acetate copolymer and ionomer and an ionomer layer. The method however has problems that since opening is conducted by tearing, the combination of resin materials is , limited to the above combination, aned the separation between the opened surface and the remaining surface cannot be made beautifully resulting in the unpleasantness of the opened surface in appearance.
In Japanese Pat. Application Laid-open No. 143,388/79 is disclosed a method of attaining easy openability by providing a resin print layer to at least one of the resin materials to be heat-sealed together to facilitate the peeling of the resin materials, and in Japanese Pat. Application Laid-open No. 203,372/1986 is disclosed a method of attaining easy openability by providing a releasing print layer to at least one of the resin materials which are to be heat-sealed together. However, neither of the methods is prevalent owing to its comprehensive process which necessarily includes printing process and adjustment of the print portion to the seal portion.
Referring to FIG. 8 and FIG. 9, which show an easily openable container having excellent seal property and openability, a container body 3 is heat-sealed with a multilayer lid member of which the inner layer is provided with notches 6' and 7' is heat-sealed to the flange portion 4 of the container body, and the delamination of the multilayer lid member is utilized when opening the container. The container however has some problems in the manufacture thereof. Firstly, the manufacture procedure of the container necessarily includes a step of providing a notch in lid member thereof, and further, owing to the thinness of the lid member, it is difficult to form the notch. Secondly, at the time of heat-sealing, the adjustment of the positions of the notch portion and the sealing portion is difficult.